knjiga solarna energija dobra

646 CADMIUM TELLURIDE SOLAR CELLS
30
25
20
15
Measured JV data
Fit with shunted
back-contact diode
327 k 318 K
309 K
306 K
Current density
[mA/cm 2 ]
10
5
0
−5
−10
−15
−20
300 K
274 K
0.4
0.5 0.6 0.7
Voltage
[V]
0.8 0.9 1.0
Figure 14.22 Measured and calculated curves for a CdTe cell with a back-contact barrier [162]
The effect of a back-contact barrier on the device band diagram was shown earlier
in Figure 14.20. The effect on measured J –V curves is illustrated in Figure 14.22. Assuming
the two depletion regions do not overlap, the two diodes can be treated as independent
circuit elements. The calculated fit to the curves, with a back-diode barrier height of
0.3 eV, is in fact quite good. The impact of the back barrier becomes larger as the
temperature is lowered. This impact is very dramatic in the first quadrant, where the
shape of the J –V curves is commonly described as “rollover” [163–165]. The degree
of “rollover” of J –V curves such as those illustrated in Figure 14.22 has been shown
to be related to the amount of copper used in the fabrication of the back contact [166].
With smaller amounts of copper, “rollover” is observed at higher temperatures implying
a larger back-contact barrier, and it has a greater impact on cell performance.
The addition of large amounts of copper to reduce the back-contact barrier, and
hence inhibit “rollover”, however, can lead to a decrease is CdTe cell stability, at least
for cells held at elevated temperatures. Several authors have seen significant performance
changes when CdTe cells were held at high temperatures (typically 60–110 ◦ C)
for extended periods of time [167–171]. Such studies, often referred to as “stress” tests,
generally first see a decrease in fill factor and next in V OC .OnlyinextremecasesisJ SC
affected. Figure 14.23 shows the illuminated J –V curves for an NREL-manufactured
CdTe cell soon after it was fabricated, and at different periods of time following light
soaking at 100 ◦ C under open-circuit bias. Such curves are typical of those seen with cells
from different manufacturers. The dark J –V curves for these and other CdTe cells also
show a progressive increase in “rollover” with continuing temperature stress. Qualitatively
similar J –V characteristics are measured for the devices of Figures 14.22 and 14.23,